Prototype Invocable Method wrapping the Salesforce Metadata API to allow management of Picklist Items via Process Builder and Visual Flow

AddPicklistItemAction.cls

/**
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**/

/**
 * This an early proof of concept for calling the Apex Metadata API from 
 *   Process Builder or indeed Visual Flow via Invocable Methods
**/
public with sharing class AddPicklistItemAction {
    
   public class Request {
	@InvocableVariable(label='Picklist Item' required=true)
	public String pickListItem;
	@InvocableVariable(label='Fully Qualified Field Name' required=true)
	public String customFieldName;
    }
    
    @InvocableMethod(
	label='Adds picklist list items to given custom fields'
	description='Will submit a request to add the given picklist items to the given custom fields')
    public static void addPickListItem(List<Request> requests) {
        // Must enqueue the work as Apex Metadata API calls our HTTP callouts
        System.enqueueJob(new DoWork(UserInfo.getSessionId(), requests));
    }
    
    /**
     * Class retrieves the custom field definitions and adds the given picklist items
     * 
     * TODO: Time did not permit the following in time for SF1 London and are still outstanding
     *  - Error reporting, email the user, or chatter post on the record?
     **/
    public class DoWork implements System.Queueable, Database.AllowsCallouts {
        
        private final String sessionId;
        
        private final List<Request> requests;
        
        public DoWork(String sessionId, List<Request> requests) {
            this.sessionId = sessionId;
            this.requests = requests;
        }
        
        public void execute(System.QueueableContext ctx) {
            
            // Metadata Service
            MetadataService.MetadataPort service = new MetadataService.MetadataPort();
            service.endpoint_x = service.endpoint_x.replace('http:', 'https:'); // Workaround to Apex MD API bug in Batch
            service.SessionHeader = new MetadataService.SessionHeader_element();
            service.SessionHeader.sessionId = sessionId;

            // Custom Fields
            Map<String, List<String>> newPickListItemsByCustomField = new Map<String, List<String>>();
            for(Request request : requests) {
		List<String> pickListItems = newPickListItemsByCustomField.get(request.customFieldName);
		if(pickListItems==null)
                    newPickListItemsByCustomField.put(request.customFieldName, pickListItems = new List<String>()); 
                pickListItems.add(request.pickListItem);
            }
            
            // Read Custom Fields
            List<MetadataService.CustomField> customFields = 
                (List<MetadataService.CustomField>) service.readMetadata('CustomField', 
                    new List<String>(newPickListItemsByCustomField.keySet())).getRecords();
            
            // Add pick list values
            for(MetadataService.CustomField customField : customFields) {
                List<String> pickListItems = newPickListItemsByCustomField.get(customField.fullName);
                for(String pickListItem : pickListItems) {
                    metadataservice.PicklistValue newPicklist = new metadataservice.PicklistValue();
                    newPicklist.fullName = pickListItem;
                    newPicklist.default_x=false;
                    customField.picklist.picklistValues.add(newPicklist);	                                        
                }
            }
                
            // Update Custom Fields and process an failed saves
            List<String> errors = new List<String>();
            for(MetadataService.SaveResult saveResult : service.updateMetadata(customFields)) {
                try {
                    handleSaveResults(saveResult);
                } catch (Exception e) {
                    errors.add(saveResult.fullName + ' : ' + e.getMessage());
                }
            }
            if(errors.size()>0) 
                throw new AddPicklistItemActionException(String.join(errors, '/n'));
        }
        
        /**
         * Example helper method to interpret a SaveResult, throws an exception if errors are found
         **/
        public void handleSaveResults(MetadataService.SaveResult saveResult)
        {
            // Nothing to see?
            if(saveResult==null || saveResult.success)
                return;
            // Construct error message and throw an exception
            if(saveResult.errors!=null) 
            {
                List<String> messages = new List<String>();
                messages.add(
                    (saveResult.errors.size()==1 ? 'Error ' : 'Errors ') + 
                        'occured processing component ' + saveResult.fullName + '.');
                for(MetadataService.Error error : saveResult.errors)
                    messages.add(
                        error.message + ' (' + error.statusCode + ').' + 
                        ( error.fields!=null && error.fields.size()>0 ? 
                            ' Fields ' + String.join(error.fields, ',') + '.' : '' ) );
                if(messages.size()>0)
                    throw new AddPicklistItemActionException(String.join(messages, ' '));
            }
            if(!saveResult.success)
                throw new AddPicklistItemActionException('Request failed with no specified error.');
        }	        
    }
    
    public class AddPicklistItemActionException extends Exception {}
}